SUNDAY, May 18, 2014 (HealthDay News) -- Vitamin D supplements do little to help control asthma, a new study found, although they might help cut the level of medication some patients need.

"Previous studies suggested that if you have asthma and low levels of vitamin D in the blood, you have worse lung function, more asthma attacks and more emergency room visits than asthma patients with higher vitamin D levels," Dr. Mario Castro, a professor of pulmonary and critical care medicine at Washington University School of Medicine in St. Louis, explained in a university news release.

So, his team decided to conduct "the first randomized controlled trial to investigate whether taking vitamin D supplements can improve asthma control," he said.

The study included more than 400 adult asthma patients at nine major medical centers in the United States. All of them had mild to moderate asthma and what was considered deficient blood levels of vitamin D.

The patients were divided into two groups: One group took an initial dose of 100,000 international units (IUs) of vitamin D3, followed by daily doses of 4,000 IUs, while the second group took a placebo.

The two groups showed no differences in all major measures of asthma control, such as the number of asthma attacks, the need for emergency care or the number of treatment failures that resulted in having to take more medication, the study found. Patients taking vitamin D also did not report improved quality of life.

The researchers did find that patients taking vitamin D were more successful in reducing their daily dosages of inhaled steroid medications. By the end of the 28-week study, the average daily doses were 111 micrograms for those in the vitamin D group versus 126 micrograms for those in the placebo group.

"The difference was small -- 15 micrograms of steroid per day -- but statistically significant," Castro said. "Over the long term, even that small amount may have an important impact on reducing side effects of inhaled steroids. Although inhaled steroids work very well in controlling asthma, patients don't like them because they cause weight gain and increase the risk of diabetes and high blood pressure. Anything we can do to reduce the amount they need is important."

The study was published online May 18 in the Journal of the American Medical Association and is also being presented the same day at the annual meeting of the American Thoracic Society, in San Diego.